Generally, a derailleur for a bicycle is so constructed that a shaft projects horizontally from a supporting member fixed to a fixing member and has at the foremost end a change-over frame swingably supported thereto, and a coiled tension spring is wound onto the shaft at its axially intermediate portion between the supporting member and the change-over frame, and is retained at one end to the supporting member and at the other end to the change-over frame to thereby apply constant tension to a driving chain guided by two pulleys on the frame.
A derailleur constucted like this, however, has the tension spring and change-over frame arranged on the shaft in series axially thereof. Hence, the shaft, which supports one pulley must have a length sufficient to include the tension spring and a bearing portion for the pulley. As a result, the derailleur as a whole has an increased lateral width.